JP2019033156A - Wavelength locker, wavelength variable laser device employing the same, and control method for wavelength variable laser device - Google Patents

Abstract

To provide a technology capable of independently performing wavelength control and temperature control by detecting deviation of a light source wavelength and temperature deviation of a wavelength filter by a simple method.SOLUTION: A wavelength locker comprises: means for branching a part of an output light from a light source into first monitor light and second monitor light; a first periodic filter to which the first monitor light is inputted; a second periodic filter to which the second monitor light is inputted; a first light-receiving element for monitoring an intensity of the light transmitted through the first periodic filter; and a second light-receiving element for monitoring an intensity of the light transmitted through the second periodic filter. The first periodic filter and the second periodic filter have such a wavelength property that the intensity of the transmitted light is periodically changed with respect to a wavelength. For the first periodic filter and the second periodic filter, shift directions of a wavelength property with respect to a temperature change are reverse to each other.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a wavelength locker, a wavelength tunable laser device using the same, and a method for controlling the wavelength tunable laser device.

  A wavelength tunable laser is mainly used as a light source of an optical communication system using wavelength multiplexing. In the wavelength tunable laser, a wavelength locker is used for precisely controlling the oscillation wavelength. In general, the wavelength locker passes light branched from the output light of the wavelength tunable laser through a wavelength filter having a periodic transmission peak such as an etalon filter, and monitors the transmitted light intensity. The wavelength is controlled by applying feedback so that the monitor value matches the value of the transmitted light intensity of the etalon at the target wavelength.

  When the temperature of a wavelength filter (etalon in the above example) having a periodic transmission characteristic is changed by the wavelength locker, the transmission characteristic of the filter shifts in parallel to the wavelength. Therefore, when the wavelength is controlled so that the monitor value of the transmitted light intensity becomes a desired value, there is a problem that the light source wavelength shifts from the target wavelength corresponding to the shift amount of the transmission characteristic of the filter due to the temperature change. .

  In the optical transmission module, the ratio of the change amount of the transmittance with respect to the wavelength change and the change amount of the transmittance with respect to the change in the external environment temperature is configured to be different between the first wavelength measuring device and the second wavelength measuring device. A wavelength monitor has been proposed (see, for example, Patent Document 1).

JP 2005-32968 A

  In the known method, the laser oscillation wavelength is stabilized by controlling the ratio of the change amount ΔIpd1 and ΔIpd2 of the monitor current from the target value at the reference temperature to always match the ratio of the temperature fluctuation coefficient of the two etalon filters. The control process is complicated. In addition, an unintended temperature shift may occur between two etalon filters having different temperature characteristics, and it is difficult to accurately extract only the influence of the two periodic filter temperature characteristic shifts.

  An object of the present invention is to provide a technique in which a wavelength filter for controlling a deviation of a light source wavelength correctly detects a light source wavelength regardless of an external temperature change.

In one embodiment, the wavelength locker is
Means for branching a part of output light from the light source into first monitor light and second monitor light;
A first periodic filter to which the first monitor light is input;
A second periodic filter to which the second monitor light is input;
A first light receiving element for monitoring the intensity of light transmitted through the first periodic filter;
A second light receiving element for monitoring the intensity of light transmitted through the second periodic filter;
The first periodic filter and the second periodic filter have wavelength characteristics in which the intensity of transmitted light periodically changes with respect to the wavelength,
The first periodic filter and the second periodic filter have opposite wavelength characteristics shift directions with respect to temperature changes.

  As one aspect, a technique is realized in which a wavelength filter for controlling the deviation of the light source wavelength correctly detects the light source wavelength regardless of an external temperature change.

It is a schematic diagram of the wavelength locker of an embodiment. It is a figure which shows the structural example of the waveguide type filter of embodiment. It is a figure which shows the relationship between the temperature characteristic of the waveguide type filter of embodiment, and optical path length. It is a figure which shows the example of the wavelength characteristic and temperature characteristic of the waveguide type filter of embodiment. It is a figure which shows an example of a characteristic when there is no wavelength shift and there is a temperature shift. It is a figure which shows another example of the wavelength characteristic and temperature characteristic of the waveguide type filter of embodiment. It is a figure which shows an example of a characteristic when there is no wavelength shift and there is a temperature shift. It is a schematic diagram of the wavelength locker of a modification. It is a figure which shows the structural example of the wavelength tunable laser apparatus using a wavelength locker.

  In the embodiment, the first filter and the second filter having a periodic transmission characteristic with respect to the wavelength are designed such that the direction of the shift of the wavelength characteristic with respect to the temperature change is opposite to each other. In the following description, a filter having periodic transmission characteristics in the wavelength region is referred to as a “periodic filter”. When the light source wavelength deviates from the target wavelength and when the temperature of the periodic filter deviates, the increase / decrease relationship between the monitor value of the transmission output of the first periodic filter and the monitor value of the transmission output of the second periodic filter is inverted. Design to do.

  For example, when both the monitor value of the transmitted light of the first periodic filter and the monitor value of the transmitted light of the second periodic filter increase in accordance with the temperature change (change in the same direction), the first change is made for the wavelength variation. The monitor value of the transmitted light of the periodic filter is increased, and the monitor value of the transmitted light of the second periodic filter is decreased (changes in the reverse direction). Thereby, the deviation of the light source wavelength from the target wavelength and the deviation of the filter temperature can be easily detected using the sum or difference of the two monitor values. Further, using the sum and difference, at least one of the light source wavelength and the filter temperature can be controlled independently of the other.

  When two filters having periodic transmission characteristics with respect to the wavelength are monolithically integrated on one chip, it is relatively easy to make the temperatures the same between the two filters. In this case, it is possible to accurately extract the difference in temperature characteristics between the filters and distinguish between the wavelength monitor and the temperature monitor.

  Drawing 1 is a mimetic diagram of wavelength locker 10 of an embodiment. A part of the output light from a light source such as a wavelength tunable laser is branched by a beam splitter (BS), for example, about 10%, and is input to two filters having periodic transmission characteristics. The two filters are a first periodic filter 11 and a second periodic filter 12. The term “transmission characteristic is periodic” means that the transmission characteristic has a wavelength characteristic in which the intensity of the output light of the filter changes periodically with respect to the wavelength. In the example of FIG. 1, the first periodic filter 11 and the second periodic filter 12 are waveguide type periodic filters formed monolithically on the same substrate 13 and formed as a filter integrated element 19 for wavelength lockers. Has been.

  The light branched by the beam splitter (BS) is branched by the optical branching waveguide 14 formed on the filter integrated element 19. The branching ratio in the optical branching waveguide 14 is, for example, 1: 1. The first periodic filter 11 is connected to one of the branch paths, and the second periodic filter 12 is connected to the other of the branch paths. The light transmitted through the first periodic filter 11 is output from the filter integrated element 19 and is incident on the first light receiving element 15 (photodiode PD1), and its light intensity is monitored. The light transmitted through the second periodic filter 12 is output from the filter integrated element 19 and is incident on the second light receiving element 16 (photodiode PD2), and its light intensity is monitored.

  As the optical branching waveguide 14, for example, a 1 × 2 multimode interference waveguide or a directional coupler waveguide whose coupling length is adjusted so that the branching ratio is 1: 1 can be used. The optical branching waveguide 14 further splits the light branched by the beam splitter (BS) into the first monitor light and the second monitor light, and supplies them to the first periodic filter 11 and the second periodic filter 12. Make each incident.

  The first periodic filter 11 is formed of an asymmetric Mach-Zhender (MZ) interferometer having a first waveguide 111 and a second waveguide 112 having different lengths. The first waveguide 111 and the second waveguide 112 extend between the 1 × 2 input-side optical coupler 113 and the 2 × 1 output-side optical coupler 114.

  The second periodic filter 12 is formed of an asymmetric MZ interferometer having a third waveguide 121 and a fourth waveguide 122 having different lengths. The third waveguide 121 and the fourth waveguide 122 extend between the 1 × 2 input-side optical coupler 123 and the 2 × 1 output-side optical coupler 124.

  As a feature of the embodiment, the first periodic filter 11 and the second periodic filter 12 have opposite peak wavelength shift directions with respect to temperature change. Thereby, the oscillation wavelength of the wavelength tunable laser and / or the temperature of the first periodic filter 11 and the second periodic filter 12 are controlled using the sum or difference of the monitor values of the light receiving element 15 and the light receiving element 16. be able to. Details of the control will be described later.

  FIG. 2 is a diagram illustrating the configuration of the first periodic filter 11 and the second periodic filter 12. Referring to FIG. 2A, the shorter waveguide of the first periodic filter 11 is referred to as a first waveguide 111, and the longer waveguide is referred to as a second waveguide 112. The shorter waveguide of the second periodic filter 12 is referred to as a third waveguide 121, and the longer waveguide is referred to as a fourth waveguide 122.

  The difference in optical path length between the first waveguide 111 and the second waveguide 112 is ΔL0. The common length of the first waveguide 111 and the second waveguide 112 excluding the optical path length difference is Lcom. The length of the shorter first waveguide 111 is Lcom, and the length of the longer second waveguide 112 is Lcom + ΔL0.

  Similarly, the optical path length difference between the third waveguide 121 and the fourth waveguide 122 is ΔL 0, and the common length of the third waveguide 121 and the fourth waveguide 122 excluding the optical path length difference is L′ com. . The length of the shorter third waveguide 121 is L'com, and the length of the longer fourth waveguide 122 is L'com + ΔL0. In FIG. 2A, the drawing is mainly focused on the first periodic filter 11, but the relationship between the two waveguides is the same in the first periodic filter 11 and the second periodic filter 12. is there.

  The optical path length difference ΔL0 of the first periodic filter 11 and the optical path length difference ΔL0 of the second periodic filter 12 are set to the same value, for example, ΔL0 = 3 mm. Since the period of the transmittance change (hereinafter simply referred to as “wavelength characteristic”) with respect to the wavelength change of the asymmetric MZ interferometer depends on the optical path length difference, a desired period can be obtained by appropriately setting ΔL0. it can. When ΔL0 = 3 mm is set, the period of the wavelength characteristic can be adjusted to about 0.8 nm (frequency is about 100 GHz).

  The first periodic filter 11 and the second periodic filter 12 are different in the shift direction of the peak wavelength of the wavelength characteristic when the temperature changes. The first periodic filter 11 is designed so that the peak wavelength shifts to the long wavelength side with respect to the temperature rise, and the second periodic filter 12 is designed to shift the peak wavelength to the short wavelength side with respect to the temperature rise. Has been. The configuration of the waveguide type periodic filter having the inverted temperature characteristics will be described in more detail.

  As shown in FIGS. 2B and 2C, the first waveguide 111 and the second waveguide 112 are designed such that changes in the equivalent refractive index with respect to changes in the waveguide temperature are different from each other. The second waveguide 112 has a waveguide cross section in which the rate of change in refractive index with respect to temperature is smaller than that of the first waveguide 111. The third waveguide 121 and the fourth waveguide 122 are different from each other in the change in the equivalent refractive index with respect to the temperature change of the waveguide, and the fourth waveguide 122 has a change in the refractive index with respect to the temperature rather than the third waveguide 121. Has a waveguide cross section with a small ratio.

Such a characteristic is, for example, a silicon fine wire waveguide in which a silicon (Si) core layer is present in the SiO ₂ clad 134 as a waveguide, and is more second than the Si core layer 135-1 of the first waveguide 111. This can be realized by narrowing the width of the Si core layer 135-2 of the waveguide 112. Similarly, the width of the Si core layer 135-2 of the fourth waveguide 122 is made narrower than the Si core layer 135-1 of the third waveguide 121. The Si core layers 135-1 and 135-2 can be formed, for example, by using an SOI (Silicon-on-Insulator) substrate as the substrate 13 and patterning the upper silicon layer by exposure and development. The SiO2 layer 132 on the silicon substrate 131 of the SOI substrate 13 is a buried oxide film and is used as a lower cladding layer.

  The rate of change in refractive index with respect to temperature changes due to the change in the width of the Si core layer because Si has a rate of change in refractive index with respect to temperature that is one order of magnitude higher than that of SiO2. When the width of the Si core layer 135-1 is large and the optical confinement is strong, the change in the equivalent refractive index with respect to the temperature change becomes large. Conversely, when the width of the Si core layer 135-2 is narrow and the light confinement is weak, the change in the equivalent refractive index with respect to the temperature change becomes small.

  By taking the configuration of FIG. 2, as an example, the rate of change in refractive index with respect to the temperature of the first waveguide 111 is + 0.0042% / ° C., and the rate of change in refractive index with respect to the temperature of the second waveguide 112 is +0.0026. Set to% / ° C. The third waveguide 121 has the same optical confinement structure as the first waveguide 111, and the ratio of the refractive index change with respect to temperature is the same as that of the first waveguide 111. The fourth waveguide 122 has the same optical confinement structure as the second waveguide 112, and the ratio of the refractive index change with respect to temperature is the same as that of the second waveguide 112. Each of the first periodic filter 11 and the second periodic filter 12 of the asymmetric MZ interferometer type is a waveguide having a large ratio of refractive index change with respect to temperature and a short waveguide, and a waveguide having a small ratio of refractive index change with respect to temperature. It is formed.

  In addition, although the example which uses the silicon | silicone thin wire | line waveguide from which optical confinement differs is mentioned as an example of a waveguide from which temperature characteristics differ here, it is not restricted to this, Two waveguides from which the temperature dependence of a refractive index change mutually differs Anything is fine. For example, the material constituting the waveguide may be a compound semiconductor or a quartz-based material, and each waveguide may be formed of two materials having different temperature characteristics in order to change the temperature characteristics between the two waveguides. .

  In order to make the degree of optical confinement different between two silicon wire waveguides forming an asymmetric MZ interferometer, the height of the Si core layer is made different, the height and the width of the Si core layer are made different, etc. May be adopted. In one of the two waveguides, at least one of the height and width of the core layer is made larger than that of the other waveguide, thereby increasing the optical confinement and increasing the refractive index change with respect to temperature change. be able to. Or you may change the shape of the waveguide used as a core between two waveguides. The first waveguide 111 and the third waveguide 121 may be formed of a rib-type silicon core having a high light confinement effect, and the second waveguide 112 and the fourth waveguide 122 may be formed of a thin-line silicon core. . Also in this case, the degree of refractive index change with respect to temperature change can be made different between the first waveguide 111 and the second waveguide 112 and between the third waveguide 121 and the fourth waveguide 122.

  FIG. 3 is a diagram illustrating the relationship between the temperature characteristics of the periodic filter according to the embodiment and the optical path length. Changes in temperature characteristics (nm / ° C) when the common optical path length Lcom is changed while maintaining the optical path length difference ΔL0 between the two waveguides with an asymmetric MZ interferometer having different temperature characteristics between the two waveguides. Show. This temperature characteristic is expressed by the shift amount of the peak wavelength of the wavelength characteristic of the asymmetric MZ interferometer with respect to the temperature change. A point where Lcom = 0 mm on the horizontal axis indicates a point where the length of the first waveguide 111 (or the third waveguide 121) is zero.

  When the length of the first waveguide 111 (or the third waveguide 121) is zero, the temperature characteristic of the asymmetric MZ interferometer reflects the temperature characteristic of the second waveguide 112 (or the fourth waveguide 122) as it is. . The ratio of the equivalent refractive index change of the second waveguide 112 (or the fourth waveguide 122) appears as it is, and the wavelength characteristic variation occurs at a rate of 0.0026% / ° C. For example, at a wavelength of 1550 nm band that is generally used in long-distance optical communication, the wavelength shifts to the long wavelength side at a rate of about 0.04 nm / ° C. While maintaining the optical path length difference ΔL0 between the first waveguide 111 and the second waveguide 112, as both the first waveguide 111 and the second waveguide 112 are lengthened, that is, as the common optical path length Lcom is lengthened, The temperature characteristics of the asymmetric MZ interferometer are reduced. When the common optical path length Lcom exceeds a certain length, the peak wavelength of the wavelength characteristic shifts to the short wavelength side as the temperature increases.

  In the example of FIG. 3, when the optical path length of the first waveguide 111 is 1 mm and the optical path length of the second waveguide 112 is 4 mm (Lcom = 1 mm, ΔL0 = 3 mm), the wavelength due to the temperature change of the first periodic filter 11 The shift amount of the characteristic is +0.032 nm / ° C. On the other hand, when the optical path length of the third waveguide 121 is 9 mm and the optical path length of the fourth waveguide 122 is 12 mm (Lcom = 9 mm, ΔL0 = 3 mm), the shift amount of the wavelength characteristic due to the temperature change of the second periodic filter 12 Becomes −0.032 nm / ° C., and a shift in the negative direction (short wavelength side) occurs. Thereby, two periodic filters with opposite temperature characteristics can be realized. Hereinafter, the principle of obtaining opposite temperature characteristics will be described.

  The optical path length difference ΔL12 between the first waveguide 111 and the second waveguide 112 when the influence of the temperature fluctuation is included is given by Expression (1). The temperature dependence of the optical path length difference ΔL12 determines the temperature dependence of the wavelength characteristics of the waveguide type periodic filter.

ΔL12 = L2 × (1 + βΔT) −L1 × (1 + αΔT) (1)
Here, L1 and L2 are the optical path lengths of the first waveguide 111 and the second waveguide 112 when the temperature is a predetermined value. α and β are ratios of changes in the equivalent refractive index with respect to the temperature of the first waveguide 111 and the second waveguide 112, both of which are positive values and α> β. As described above, the ratio of the refractive index change with respect to the temperature change of the first waveguide 111 is designed to be larger than the refractive index change with respect to the temperature change of the second waveguide 111.
The optical path lengths L1 and L2 are set such that the difference between L2 and L1 (L2−L1 = ΔL0) is a fixed value from the viewpoint of setting the filter period of the asymmetric MZ interferometer to a desired value. In this example, ΔL0 = 3 mm. The first waveguide 111 and the second waveguide when there is a temperature variation with respect to the common optical path length Lcom when both the first waveguide 111 and the second waveguide 112 are lengthened while the optical path length difference ΔL0 is fixed. The optical path length difference ΔL12 of 112 is
ΔL12 = (ΔL0 + Lcom) × (1 + βΔT) −Lcom × (1 + αΔT)
= ΔL0 + {βΔL− (α−β) Lcom} ΔT
= ΔL0 + CΔT (2)
It is expressed. Here, C is a coefficient of temperature change of the optical path length difference of the asymmetric MZ interferometer, and from Equation (2),
C = βΔL− (α−β) Lcom (3)
It is expressed.

The coefficient C decreases as both the first waveguide 111 and the second waveguide are lengthened, that is, as Lcom is increased.
Lcom> βΔL / (α−β) (4)
Under such conditions, the change in the optical path length with respect to temperature becomes negative. The wavelength characteristic of the periodic filter 11 formed by the asymmetric MZ interferometer depends on the change of the optical path length difference ΔL12, and shifts to the long wavelength side when the optical path length difference ΔL12 increases, and shifts to the short wavelength side when it decreases. Therefore, by adjusting the value of the common optical path length Lcom as shown in FIG. 3, the direction of the wavelength shift due to temperature can be reversed, and the waveguide type periodic filters 11 and 12 whose temperature characteristics are mutually inverted are realized. be able to.

  Next, a wavelength locker using the first periodic filter 11 and the second periodic filter 12 whose temperature characteristics are inverted from each other and wavelength control thereof will be described.

  FIG. 4 shows an example of the wavelength characteristics of the waveguide-type first periodic filter 11 and the second periodic filter 12 of the embodiment. In this example, the slope of the transmittance (sine curve labeled “monitor 1”) with respect to the wavelength of the first periodic filter 11 indicated by a thick solid line is positive in the vicinity of the target wavelength of the laser. The slope of the transmittance (sine curve labeled “monitor 2”) with respect to the wavelength of the second periodic filter 12 indicated by a thin solid line is negative.

  The monitor values of the first light receiving element 15 (PD1) and the second light receiving element 16 (PD2) correspond to the transmittances of the first periodic filter 11 and the second periodic filter 12, respectively. However, the first light receiving element 15 is positive, and the second light receiving element 16 is negative. As shown in FIG. 4A, the wavelength at the point where the transmittances of the first periodic filter 11 and the second periodic filter 12 cross each other is adjusted to match the target wavelength at a predetermined temperature. In other words, the wavelength at which the monitor value at the first light receiving element 15 and the monitor value at the second light receiving element 16 cross each other is adjusted to coincide with the target wavelength. This wavelength adjustment can be realized, for example, by arranging a phase shifter using a local heater in a part of the asymmetric MZ interferometer and adjusting the amount of current injected into the phase shifter.

  In FIG. 4B, the first periodic filter 11 and the second periodic filter are adjusted to the above wavelength relationship, and the temperature of the filter integrated element 19 for wavelength locker is kept at a predetermined temperature, The case where the wavelength is shifted from the target wavelength toward the long wavelength side is shown. In this case, the monitor value (monitor 1) of the first light receiving element 15, that is, the transmittance of the first periodic filter 11, increases, and the monitor value (monitor 2) of the second light receiving element 16, that is, the second period. The transmittance of the filter 12 decreases by the same amount. When the wavelength of the light source deviates, the difference between the monitor value of the first light receiving element 15 and the monitor value of the second light receiving element changes, and the sum becomes substantially constant. FIG. 4C shows a case where the temperature of the filter integrated element 19 for wavelength locker is higher than a predetermined temperature from the state of FIG. 4B. In this case, the wavelength characteristic of the first periodic filter 11 is shifted from the thick dotted line state to the long wavelength side, and the wavelength characteristic of the second periodic filter 12 is shifted from the thin dotted line state to the short wavelength side by the same amount. To do. As a result, the wavelength at which the wavelength characteristics of the first periodic filter 11 and the second periodic filter 12 cross does not change and is maintained at the target wavelength. At the target wavelength, the monitor value of the first light receiving element 15 and the monitor value of the second light receiving element 16 both decrease at the same rate. Therefore, the difference between the monitor value of the first light receiving element 15 and the monitor value of the second light receiving element becomes zero when the filter temperature changes when the wavelength of the light source matches the target wavelength. By controlling the wavelength of the light source so that the difference between the monitor value of the first light receiving element 15 and the monitor value of the second light receiving element 16 becomes zero, it is possible to accurately match the target wavelength regardless of the temperature.

  Paying attention to the change in the monitor value with respect to the temperature, if the sum of the monitor value of the first light receiving element 15 and the monitor value of the second light receiving element 16 is taken when the temperature changes, the sum value becomes the deviation of the light source wavelength. Does not depend on much, but changes strongly depending on temperature changes. This is because by taking the sum of the monitor values, the effects of wavelength shifts cancel each other due to the reversal of the wavelength characteristics in the vicinity of the target wavelength of the two periodic filters. By taking the sum of the two monitor values, even if the wavelength of the light source is deviated from the target wavelength, only the influence of temperature fluctuations can be extracted.

  As in the embodiment, the first periodic filter 11 and the second periodic filter 12 having the temperature characteristics inverted from each other are used, and the monitor value of the first light receiving element 15 and the second light receiving element 16 are used. By taking the sum or difference of these monitor values, it is possible to accurately monitor the deviation of the wavelength from the target wavelength and the deviation of the filter temperature from the predetermined temperature by a simple method. The light source wavelength and / or the temperature of the filter integrated element 19 for wavelength locker can be accurately controlled independently of each other by simple monitor value calculation as compared with the conventional method.

  FIG. 5 shows the filter characteristics when the laser oscillation wavelength matches the target wavelength and the temperature of the filter deviates. FIG. 5A shows a state in which there is no wavelength shift or temperature shift as in FIG. 4A. Focusing on the change in the monitor value with respect to the temperature, when the temperature changes as shown in FIG. 5B, the monitor value decreases for both the first and second photodiodes (for example, the monitor value decreases from a monitor value a to b). By taking the sum of the monitor values, the change in temperature can be extracted. Further, even when the wavelength is further shifted in a state where the temperature is shifted, the increase / decrease in the monitor values of the first and second photodiodes due to the wavelength shift is canceled by taking the sum. Therefore, by taking the sum, it is possible to extract mainly the influence of temperature fluctuations even if the wavelength is deviated from the target wavelength.

  FIG. 6 shows another example of the wavelength characteristics of the first periodic filter 11 and the second periodic filter 12 of the waveguide type according to the embodiment. In FIG. 6A, the wavelength characteristics of the first periodic filter 11 and the second periodic filter 12 overlap at a predetermined temperature, and the transmittance at the target wavelength is adjusted to 50%. As in FIG. 4, the monitor value of the first light receiving element 15 and the monitor value of the second light receiving element 16 correspond to the transmittances of the first periodic filter 11 and the second periodic filter 12, respectively. At temperature, the first periodic filter 11 and the second periodic filter 12 have the same change in monitor value with respect to wavelength.

  In FIG. 6B, when the wavelength of the light source shifts while the wavelength locker filter integrated element 19 is kept at a predetermined temperature, the first periodic filter 11 and the second periodic filter 12 are changed according to the wavelength shift. The transmittance of the first light receiving element 15 and the second light receiving element 16 changes accordingly. Therefore, if the light source wavelength is controlled so that the monitor values of both the first light receiving element 15 and the second light receiving element 16 are equivalent to the transmittance of 50%, the light source can be obtained in the same manner as a general wavelength locker. It is possible to control the wavelength of (laser). Note that, at the “predetermined temperature” described above, the wavelength characteristics of the first periodic filter 11 and the second periodic filter 12 are the same, so the difference between the monitor values is zero regardless of the wavelength shift.

  In FIG. 6C, when a temperature deviation further occurs from the state of FIG. 6B, the monitor output of the first periodic filter 11 shifts to the long wavelength side, and the monitor output of the second periodic filter 12 Shifts to the short wavelength side. As a result, the wavelength characteristics are separated between the first periodic filter 11 and the second periodic filter 12. At the target wavelength, the monitor value of the first light receiving element 15 decreases and the monitor value of the second light receiving element 16 increases. Since the monitor output of the first light receiving element 15 and the monitor output of the second light receiving element 16 move in opposite directions, when they are summed, they cancel each other and are almost constant regardless of the presence or absence of temperature deviation. Takes the value of Therefore, the wavelength of the light source (laser) is taken so that the sum of the two monitor values is taken regardless of the temperature, and the average value of the monitor values becomes a value corresponding to the target transmittance (in this example, the transmittance is 50%). Is controlled, the light source wavelength can be adjusted to the target wavelength.

  When the temperature of the filter integrated element 19 for wavelength locker changes, the wavelength characteristics of the first periodic filter 11 and the second periodic filter 12 are shifted. Therefore, if the difference between the respective monitor values is taken, the temperature shift Can be monitored.

  FIG. 7 shows the filter characteristics when the laser oscillation wavelength matches the target wavelength and the temperature of the filter deviates. FIG. 7A shows a state in which there is no wavelength shift or temperature shift as in FIG. 6A. The difference between the two monitor values at this time is zero. As shown in FIG. 7B, when the temperature of the wavelength locker filter element changes, a shift occurs in the wavelength characteristics of the first and second waveguide type periodic filters, and one monitor value increases. The other monitor value decreases. Therefore, if the difference between the monitor values is taken, the temperature deviation can be monitored. If the temperature is controlled so that the difference between the monitor values becomes 0, the temperature of the wavelength locker filter can be adjusted to a desired value.

  Whether the wavelength characteristics of the first periodic filter 11 and the second periodic filter 12 are set to be the same on the assumption that the direction of the change of the wavelength characteristic with respect to the temperature change is reversed (FIGS. 6 and 7). The calculation of the monitor value varies depending on whether the inclination of is set in the reverse direction (FIGS. 4 and 5).

  4 and 5, the wavelength shift is detected by the difference between the two monitor values, and the temperature shift is detected by the sum of the two monitor values. In the examples of FIGS. 6 and 7, the wavelength shift is detected by the two monitor values. The temperature difference is detected by the difference between the two monitor values. 4 to 7, by taking a simple sum and difference of the monitor values of the first light receiving element 15 and the second light receiving element 16, the wavelength deviation from the target wavelength and the predetermined temperature Temperature deviation can be monitored with high accuracy. Compared with the conventional configuration, the wavelength control and the temperature control of the filter integrated element 19 can be accurately and independently performed with a simple calculation of the monitor value.

  In the example of FIGS. 4 to 7, the wavelength characteristic shift with respect to the temperature change of the first periodic filter 11 and the second periodic filter 12 shows the same shift amount in the reverse direction, but completely the same. Even when it is not the shift amount, it is sufficiently effective. Since the shift of the wavelength characteristic due to the temperature change always works in the direction in which the first periodic filter 11 and the second periodic filter 12 cancel each other, the oscillation wavelength with respect to the temperature change of the wavelength locker is compared with the general wavelength locker. This is because the influence of deviation can be reduced.

<Modification>
FIG. 8 is a schematic diagram of a modified wavelength locker 10A. The wavelength locker 10A includes a third light receiving element 17 (PD3) and a beam splitter (BS2) for branching the laser light to the third light receiving element 17 in addition to the configuration of the wavelength locker 10 of FIG. The same components as those in FIG. The third light receiving element 17 detects the light intensity proportional to the output light itself of the light source without passing through a wavelength-dependent filter or the like.

  Using the monitor output from the third light receiving element 17, the monitor output of the first light receiving element 15 that detects the transmitted light of the first periodic filter 11 and the first output that detects the transmitted light of the second periodic filter 12. The monitor output of the two light receiving elements 16 can be normalized. Thereby, even when the output intensity of the laser fluctuates, a stable monitor value can be obtained, and the wavelength control is stabilized. Even in the modified example, the first periodic filter 11 and the second periodic filter 12 of the waveguide type are designed so that the shift directions of the wavelength characteristics with respect to the temperature change are opposite to each other. The wavelength shift from the target wavelength and the temperature shift from the predetermined temperature can be detected with high accuracy.

<Wavelength tunable laser device>
FIG. 9 is a schematic diagram of the wavelength tunable laser device 1 according to the embodiment. The wavelength tunable laser device 1 includes a wavelength tunable laser light source, a wavelength locker 10, and a controller 2. The main part of the wavelength locker 10 is formed as a filter integrated element 19 and is disposed on the Peltier element 4 for temperature control. Outputs of the first light receiving element 15 and the second light receiving element 16 of the wavelength locker 10 are connected to an input of the controller 2.

  Based on the monitor values of the first light receiving element 15 and the second light receiving element 16, the controller 2 determines the wavelength of the wavelength tunable laser light source 3 and the temperature of the Peltier element 4, that is, the first periodic filter 11 and the second periodic filter 11. The temperature of the periodic filter 12 is controlled. The controller 2 includes a processor 200 and a memory 205, and the processor 200 performs calculations using a work area in the memory 205.

  The processor 200 includes an adder 201, a subtracter 202, a wavelength control unit 203, and a temperature control unit 204. The adder 201 calculates the sum of the monitor value of the first light receiving element 15 and the monitor value of the second light receiving element 16. The subtractor 202 calculates the difference between the monitor value of the first light receiving element 15 and the monitor value of the second light receiving element 16. The wavelength control unit 203 adjusts the wavelength control parameter of the wavelength tunable laser light source 3 based on the outputs of the adder 201 and the subtracter 202, and controls the light source wavelength to the target wavelength. When a temperature shift is detected, the temperature control unit 204 controls the amount of current injected into the Peltier element 4 so that the temperature of the filter integrated element 19 is maintained at a predetermined temperature. Control parameters for the wavelength tunable laser light source 3 and control parameters for the Peltier element 4 are stored in the memory 205.

  When the wavelength characteristics of the first periodic filter 11 and the second periodic filter 12 are set as shown in FIG. 4A (and FIG. 5A) (the gradient of the monitor intensity with respect to the wavelength change is reversed). The wavelength control unit 203 controls the wavelength of the wavelength tunable laser light source 3 so that the difference between the monitor values output from the subtracter 202 approaches zero. Thereby, the wavelength of the wavelength tunable laser light source 3 is adjusted to the target wavelength. The temperature control unit 204 adjusts the temperature of the filter integrated element 19 by controlling the current amount of the Peltier element 4 so that the sum of the monitor values output from the adder 201 becomes a predetermined value.

  When the wavelength characteristics of the first periodic filter 11 and the second periodic filter 12 are set to the same characteristics as shown in FIGS. 6 and 7, the wavelength control unit 203 outputs the monitor value output from the adder 201. The wavelength of the wavelength tunable laser light source 3 is controlled so that the sum of the values becomes a predetermined value. Thereby, the light source wavelength is controlled to the target wavelength. The temperature control unit 204 adjusts the temperature of the filter integrated element 19 by controlling the current amount of the Peltier element 4 so that the difference between the monitor values output from the subtracter 202 approaches zero.

  The temperature control of the filter integrated element 19 (or the Peltier element 4) is not necessarily performed. Since the wavelength locker 10 of the embodiment is designed in a direction that always cancels the shift of the wavelength characteristic with respect to the temperature change even when there is a certain temperature fluctuation, the wavelength of the wavelength tunable laser light source 3 is controlled to the target wavelength. Is possible. By controlling the temperature of the filter integrated element 19, the oscillation wavelength of the light source can be controlled with higher accuracy.

  Although the invention has been described based on the specific embodiments, the present invention is not limited to the above embodiments. For example, an example in which 2 × 1 couplers are used as the output-side optical couplers 114 and 124 of a waveguide type periodic filter formed by an asymmetric MZ interferometer has been disclosed, but this is disclosed in a publicly known document (for example, JP-A-2015-6854). A 90 ° hybrid waveguide may be used instead of the 2 × 1 coupler by adopting the structure described above. A right-angle prism mirror may be used instead of the beam splitter. In addition to the filter integrated element 19, the first light receiving element 15 and the second light receiving element 16 may be arranged on the Peltier element 4. Instead of the wavelength locker 10 of FIG. 9, the wavelength locker 10A of FIG. 8 may be used. The optical path length difference ΔL0 between the two waveguides in each of the first periodic filter 11 and the second periodic filter 12 is not limited to 3 mm, and is appropriately selected according to the frequency band to be used or the grid interval. Also in this case, the common optical path length Lcom is set so that the change in the wavelength characteristic with respect to the temperature change is reversed between the two periodic filters.

The following notes are presented for the above explanation.
(Appendix 1)
Means for branching a part of output light from the light source into first monitor light and second monitor light;
A first periodic filter to which the first monitor light is input;
A second periodic filter to which the second monitor light is input;
A first light receiving element for monitoring the intensity of light transmitted through the first periodic filter;
A second light receiving element for monitoring the intensity of light transmitted through the second periodic filter;
Have
The first periodic filter and the second periodic filter have wavelength characteristics in which the intensity of transmitted light periodically changes with respect to the wavelength,
The wavelength locker, wherein the first periodic filter and the second periodic filter have opposite wavelength characteristics shift directions with respect to temperature change.
(Appendix 2)
The wavelength locker according to appendix 1, wherein the first periodic filter and the second periodic filter are formed of an asymmetric Mach-Zehnder interferometer.
(Appendix 3)
The first periodic filter has a first waveguide and a second waveguide longer than the first waveguide,
The second periodic filter has a third waveguide and a fourth waveguide longer than the third waveguide,
The optical path length difference between the first waveguide and the second waveguide and the optical path length difference between the third waveguide and the fourth waveguide are the same,
The wavelength locker according to appendix 2, wherein the optical path length of the first waveguide is different from the optical path length of the third waveguide.
(Appendix 4)
The common optical path length of the first waveguide and the second waveguide, and the common optical path length of the third waveguide and the fourth waveguide are the temperature changes of the first periodic filter and the second periodic filter. The wavelength locker according to appendix 3, wherein the shift directions of the wavelength characteristics with respect to are opposite to each other and have the same shift amount.
(Appendix 5)
The rate of change of the equivalent refractive index with respect to the temperature change of the first waveguide is larger than the rate of change of the equivalent refractive index with respect to the temperature change of the second waveguide,
The wavelength locker according to appendix 3, wherein a rate of change of the equivalent refractive index with respect to a temperature change of the third waveguide is larger than a rate of change of the equivalent refractive index with respect to a temperature change of the fourth waveguide.
(Appendix 6)
The second waveguide is longer than the first waveguide, and the width of the second waveguide is narrower than the width of the first waveguide;
The wavelength locker according to any one of appendices 3 to 5, wherein the fourth waveguide is longer than the third waveguide, and the width of the fourth waveguide is narrower than the width of the third waveguide. .
(Appendix 7)
At the target wavelength, the direction of change of the monitor value of the first light receiving element with respect to the wavelength is opposite to the direction of change of the monitor value of the second light receiving element with respect to the wavelength. The wavelength locker according to appendix 1, wherein the monitor value and the monitor value of the second light receiving element are the same.
(Appendix 8)
The wavelength locker according to appendix 1, wherein the change of the output value of the first light receiving element with respect to the wavelength at the target wavelength is the same as the change of the output value of the second light receiving element with respect to the wavelength.
(Appendix 9)
The wavelength locker according to any one of appendices 1 to 8, wherein the first periodic filter and the second periodic filter are formed in a monistic manner on the same substrate.
(Appendix 10)
A laser light source;
A wavelength locker into which a part of the output light of the laser light source is input;
In a wavelength tunable laser device having a controller that controls the wavelength of the laser light source to a target wavelength based on the output of the wavelength locker,
The wavelength locker includes means for branching a part of output light of the laser light source into first monitor light and second monitor light, a first periodic filter to which the first monitor light is input, A second periodic filter to which the second monitor light is input; a first light receiving element for monitoring the intensity of the light transmitted through the first periodic filter; and the intensity of the light transmitted through the second periodic filter. A second light receiving element for monitoring
The first periodic filter and the second periodic filter have wavelength characteristics in which the intensity of transmitted light periodically changes with respect to the wavelength,
In the first periodic filter and the second periodic filter, the shift directions of the wavelength characteristics with respect to the temperature change are opposite to each other,
The tunable laser apparatus, wherein the controller controls a light source wavelength to the target wavelength by using a sum or a difference between a monitor value of the first light receiving element and a monitor value of the second light receiving element.
(Appendix 11)
At the target wavelength, the direction of change of the monitor value of the first light receiving element with respect to the wavelength is opposite to the direction of change of the monitor value of the second light receiving element with respect to the wavelength. The monitor value and the monitor value of the second light receiving element are the same,
The wavelength according to appendix 10, wherein the controller controls the wavelength of the laser light source so that a difference between a monitor value of the first light receiving element and a monitor value of the second light receiving element approaches zero. Variable laser device.
(Appendix 12)
A temperature adjusting element for adjusting the temperature of the first periodic filter and the second periodic filter;
Further comprising
The controller 11 controls the temperature of the temperature adjusting element so that a sum of a monitor value of the first light receiving element and a monitor value of the second light receiving element becomes a predetermined value. The wavelength tunable laser device described in 1.
(Appendix 13)
At the target wavelength, the change of the monitor value of the first light receiving element with respect to the wavelength is the same as the change of the monitor value of the second light receiving element with respect to the wavelength.
The controller 10 controls the wavelength of the laser light source so that a sum of a monitor value of the first light receiving element and a monitor value of the second light receiving element becomes a predetermined value. Tunable laser device.
(Appendix 14)
A temperature adjusting element for adjusting the temperature of the first periodic filter and the second periodic filter;
Further comprising
Item 14. The supplementary note 13, wherein the controller controls the temperature of the temperature adjustment element so that a difference between a monitor value of the first light receiving element and a monitor value of the second light receiving element approaches zero. Tunable laser device.
(Appendix 15)
A method of controlling a wavelength tunable laser device having a wavelength locker,
The wavelength locker has a wavelength characteristic in which the intensity of transmitted light periodically changes with respect to a wavelength, and a first periodic filter having a temperature characteristic in which a shift direction of the wavelength characteristic with respect to a temperature change is reversed, and a second Place a periodic filter,
Monitoring the transmission output of the first periodic filter and the transmission output of the second periodic filter;
A wavelength tunable laser device that controls a light source wavelength to a target wavelength by using a sum or difference of a first monitor value of the first periodic filter and a second monitor value of the second periodic filter Control method.
(Appendix 16)
In the target wavelength, the direction of change of the first monitor value with respect to the wavelength is opposite to the direction of change of the second monitor value, and the first monitor value and the second monitor value are the same. Adjusting the first periodic filter and the second periodic filter;
16. The method of controlling a wavelength tunable laser device according to appendix 15, wherein the light source wavelength is controlled so that a difference between the first monitor value and the second monitor value approaches zero.
(Appendix 17)
The temperature of the first periodic filter and the second periodic filter is controlled so that the sum of the first monitor value and the second monitor value becomes a predetermined value. Control method of wavelength tunable laser device.
(Appendix 18)
Adjusting the first periodic filter and the second periodic filter so that the change of the first monitor value with respect to the wavelength and the change of the second monitor value with respect to the wavelength are the same at the target wavelength;
16. The method of controlling a wavelength tunable laser device according to appendix 15, wherein the light source wavelength is controlled so that a sum of the first monitor value and the second monitor value becomes a predetermined value.
(Appendix 19)
The wavelength tunable according to appendix 18, wherein temperatures of the first periodic filter and the second periodic filter are controlled so that a difference between the first monitor value and the second monitor value approaches zero. Control method of laser apparatus.

DESCRIPTION OF SYMBOLS 1 Wavelength variable laser apparatus 2 Controller 3 Wavelength variable laser light source 4 Peltier element 10, 10A, wavelength locker 11 1st periodic filter 12 2nd periodic filter 13 Substrate 14 Branching waveguide 15 1st light receiving element 16 2nd light reception Element 17 Third light receiving element 19 Filter integrated element 111 First waveguide 112 Second waveguide 113, 123 Input side optical coupler 114, 124 Output side optical coupler 121 Third waveguide 122 Fourth waveguide 200 Processor 201 Adder 202 Subtractor 203 Wavelength control unit 204 Temperature control unit 205 Memory

Claims (10)

Means for branching a part of output light from the light source into first monitor light and second monitor light;
A first periodic filter to which the first monitor light is input;
A second periodic filter to which the second monitor light is input;
A first light receiving element for monitoring the intensity of light transmitted through the first periodic filter;
A second light receiving element for monitoring the intensity of light transmitted through the second periodic filter;
Have
The first periodic filter and the second periodic filter have wavelength characteristics in which the intensity of transmitted light periodically changes with respect to the wavelength,
The wavelength locker, wherein the first periodic filter and the second periodic filter have opposite wavelength characteristics shift directions with respect to temperature change.   The wavelength locker according to claim 1, wherein the first periodic filter and the second periodic filter are formed of an asymmetric Mach-Zehnder interferometer. The first periodic filter has a first waveguide and a second waveguide longer than the first waveguide,
The second periodic filter has a third waveguide and a fourth waveguide longer than the third waveguide,
The optical path length difference between the first waveguide and the second waveguide and the optical path length difference between the third waveguide and the fourth waveguide are the same,
The wavelength locker according to claim 2, wherein the optical path length of the first waveguide and the optical path length of the third waveguide are different. The second waveguide is longer than the first waveguide, and the width of the second waveguide is narrower than the width of the first waveguide;
4. The wavelength locker according to claim 3, wherein the fourth waveguide is longer than the third waveguide, and the width of the fourth waveguide is narrower than the width of the third waveguide.   At the target wavelength, the direction of change of the monitor value of the first light receiving element with respect to the wavelength is opposite to the direction of change of the monitor value of the second light receiving element with respect to the wavelength. The wavelength locker according to claim 1, wherein a monitor value and a monitor value of the second light receiving element are the same.   2. The wavelength locker according to claim 1, wherein, at a target wavelength, a change in the output value of the first light receiving element with respect to the wavelength is the same as a change in the output value of the second light receiving element with respect to the wavelength. A laser light source;
A wavelength locker into which a part of the output light of the laser light source is input;
In a wavelength tunable laser device having a controller that controls the wavelength of the laser light source to a target wavelength based on the output of the wavelength locker,
The wavelength locker includes means for branching a part of output light of the laser light source into first monitor light and second monitor light, a first periodic filter to which the first monitor light is input, A second periodic filter to which the second monitor light is input; a first light receiving element for monitoring the intensity of the light transmitted through the first periodic filter; and the intensity of the light transmitted through the second periodic filter. A second light receiving element for monitoring
The first periodic filter and the second periodic filter have wavelength characteristics in which the intensity of transmitted light periodically changes with respect to the wavelength,
In the first periodic filter and the second periodic filter, the shift directions of the wavelength characteristics with respect to the temperature change are opposite to each other,
The tunable laser apparatus, wherein the controller controls a light source wavelength to the target wavelength by using a sum or a difference between a monitor value of the first light receiving element and a monitor value of the second light receiving element. At the target wavelength, the direction of change of the monitor value of the first light receiving element with respect to the wavelength is opposite to the direction of change of the monitor value of the second light receiving element with respect to the wavelength. The monitor value and the monitor value of the second light receiving element are the same,
8. The controller according to claim 7, wherein the controller controls the wavelength of the laser light source so that a difference between a monitor value of the first light receiving element and a monitor value of the second light receiving element approaches zero. Tunable laser device. At the target wavelength, the change of the monitor value of the first light receiving element with respect to the wavelength is the same as the change of the monitor value of the second light receiving element with respect to the wavelength.
8. The controller according to claim 7, wherein the controller controls a wavelength of the laser light source so that a sum of a monitor value of the first light receiving element and a monitor value of the second light receiving element becomes a predetermined value. The tunable laser device described. A method of controlling a wavelength tunable laser device having a wavelength locker,
The wavelength locker has a wavelength characteristic in which the intensity of transmitted light periodically changes with respect to a wavelength, and a first periodic filter having a temperature characteristic in which a shift direction of the wavelength characteristic with respect to a temperature change is reversed, and a second Place a periodic filter,
Monitoring the transmission output of the first periodic filter and the transmission output of the second periodic filter;
A wavelength tunable laser device that controls a light source wavelength to a target wavelength by using a sum or difference of a first monitor value of the first periodic filter and a second monitor value of the second periodic filter Control method.